The House as a System: Heat, Air, and Moisture
Establish systems thinking: heat, air, and moisture flows are coupled, so changing one often changes the others. This session sets up the logic behind control layers and unintended consequences.
How this fits in the series
Builds on:
P3 (loads) and
P4 (failure mechanisms)
Leads to:
P6–P11
Core concepts and execution implications
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Heat, air, and moisture are coupled.
- Can evaluate measures for side effects, not single-metric wins.
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Air movement often dominates moisture transport.
- Can treat airtightness and pressure control as moisture controls.
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System boundaries matter.
- Can define conditioned space and pressure regimes explicitly.
Connections
- Performance framework: 82 edges across 8 degradation/mitigation chains (systems interactions)
- Cross-series: A5 House Size & Design (size and geometry affect HAM interactions)
- Explore in Performance Framework →
What good looks like
A "good" enclosure isn't one that's perfect—it's one that is predictable. We want assemblies that:
- Limit wetting and tolerate the wetting that still happens.
- Manage drying (in at least one reliable direction, in this climate).
- Control airflow/pressure so air doesn't carry moisture into risky places.
- Control temperatures so surfaces don't spend long periods below dew point.
- Stay robust against normal occupant behavior, minor defects, and aging.
Explore in PF: These five qualities map to the drying, wetting, and pressure edges in the framework—each is a chain you can trace from load to control.
Where things go wrong
These are the patterns that show up when heat/air/moisture aren't treated as a coupled system.
Coupled-flow failure patterns you can name in conversation: "Warm air finds cold surface" (exfiltration → condensation), "Pressure does the damage" (negative pressure pulls in bad air), "Drying path got blocked" (upgrade removes drying direction), and "Small leak + time" (minor leak + slow drying = rot). Examples:
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Exfiltration into a cold roof zone
Air leakage carries moisture to a cold surface; the roof "mysteriously" molds or frosts even with decent insulation.
Pattern: warm air finds cold surface
Field check: blower door with smoke at ceiling plane; thermal scan roof deck for cold-weather moisture -
Basement/garage odor and humidity complaints
Pressure imbalances or return leaks pull contaminants/moisture into living space; symptoms look like "IAQ" but root cause is airflow.
Pattern: pressure does the damage
Field check: measure pressure across garage/basement boundary with HVAC running; inspect return ducts for leaks -
"We sealed it up and now it's wet"
Air sealing is good—until it removes a drying mechanism without adding a controlled one (ventilation, dehumidification, or assembly changes).
Pattern: drying path got blocked
Field check: verify at least one drying direction remains open; confirm ventilation sized for new tightness level
Explore in PF: Each pattern corresponds to a degradation chain in the framework. Trace the edge from the load to its uncontrolled outcome to see the full failure path.